Regardless of the uniquely nourishing properties of meat, there are people who just don't want to eat meat. There are also people who might want to abstain from it periodically. Sometimes in the blogosphere it might seem as if it's a choice between steak and absurdly dysfunctional raw vegan diets. This is a false dichotomy perpetuated by people who have a bizarre distorted views about animals.
Enter animals that definitely don't have feelings or interests beyond very basic biological urges. In his excellent essay on the idea of oysters being OK for people who otherwise would be vegan, Christopher Cox says:
But what if we could find an animal that thrived in a factory-farm cage, one that subsisted on nutrients plucked from the air and that was insensate to the slaughterhouse blade? Even if that animal looked like a bunny rabbit crossed with a puppy, it would be A-OK to hack it into pieces for your dinner plate. Luckily for those of us who still haven't gotten over the death of Bambi's mother, the creature I'm thinking of is decidedly less cuddly. Biologically, oysters are not in the plant kingdom, but when it comes to ethical eating, they are almost indistinguishable from plants. Oyster farms account for 95 percent of all oyster consumption and have a minimal negative impact on their ecosystems; there are even nonprofit projects devoted to cultivating oysters as a way to improve water quality. Since so many oysters are farmed, there's little danger of overfishing. No forests are cleared for oysters, no fertilizer is needed, and no grain goes to waste to feed them—they have a diet of plankton, which is about as close to the bottom of the food chain as you can get. Oyster cultivation also avoids many of the negative side effects of plant agriculture: There are no bees needed to pollinate oysters, no pesticides required to kill off other insects, and for the most part, oyster farms operate without the collateral damage of accidentally killing other animals during harvesting. (Relatedly, although it's possible to collect wild oysters sustainably, the same cannot be said for other bivalves like clams and mussels. These are often dredged from the seabed, disrupting an entire ecosystem. For that reason, it's best to avoid them.)
If you are slightly less sensitive as least less than David Foster Wallace (as I am), you might want to also consider the lobster, as lobster harvesting has also become much more sustainable. Other shellfish options are a bit more murky. Good shrimp is exceedingly hard to find, most of it raised in polluted cesspools in Asia or caught from a now-destroyed Gulf. Clams are often harvested through dredging, which destroys the sea floor. Overall, while I do have some access to things like local diver scallops, I have found that even though I live on an island, meat is much easier to source.
The role of shellfish in human evolution is controversial, but they are very easily gathered and treasured by many cultures. In Human Brain Evolution: The Influence of Freshwater and Marine Food Resources there is a wonderful table, which is "amount of each major food groups required to meet the daily requirement for five brain-selective minerals- iodine, iron, zinc, copper, and selenium." You only need 900 g shellfish vs. 5000 grams meat vs. 9300 grams fruit vs. 47,000 grams cow's milk. The book notes that in evolution "a combination of shellfish, fish, nuts, eggs, meat, and fruit seems to be more likely than exclusive consumption of one food group..." Either way, you don't need much shellfish in your diet to make a difference between a mediocre vegetarian/vegan diet and an excellent omnivorous one. Besides those minerals so important in brain development, shellfish also provide DHA, B12, and taurine, also very important for the brain.
I have occasionally relied primarily on a shellfish-based diet and the major limit to this is shellfish don't have much in the way of calories. Eat just oysters for dinner and you will be hungry. You also need to rely on a variety of other nourishing calorie sources like coconut, roots, and fruit. An excellent staple is shellfish boiled in coconut milk with some roots. Mmm.